1. Field of the Invention
The present invention relates to a battery management control system for controlling input and output of a battery used as a power supply of, for example, an electric vehicle in association with deterioration of the battery.
2. Description of the Related Art
Electric vehicles install therein batteries as power supplies for driving. The battery includes a plurality of battery cells, a battery case which stores the battery cells and the like.
Lithium ion secondary batteries are used as an example of a battery. A battery includes a plurality of secondary batteries (battery cells), and a voltage sensor and a temperature sensor are provided for each battery cell. The voltage sensors and temperature sensors are connected to a vehicle control unit, and the vehicle control unit detects output voltage values and input voltage values of the respective battery cells, as well as temperatures of the respective battery cells.
In the battery that is configured as has been described above, its maximum output value (W) and maximum input value (W) are changed by its charging rate. Note that the maximum input value means a maximum value of electric power that can be inputted into each battery cell to charge the battery cell by a regenerative brake system when the speed of an electric vehicle is reduced.
The maximum output value of each battery cell increases more as the charging rate of the battery or the charging rate of the battery cell becomes closer to full charge. When the charging rate becomes smaller, the maximum output value becomes smaller. On the contrary, the maximum input value of each battery cell becomes smaller as the charging rate of the battery cell becomes closer to full charge, and when the charging rate becomes smaller, the maximum input value becomes larger.
The maximum output value and maximum input value of the battery are stored on the maps. The maximum output electric power value of the battery shown on the map means a total sum of respective maximum electric power values that the battery cells installed in the battery can output. The maximum input electric power value of the battery shown on the map means a total sum of respective maximum electric power values that can be inputted into the battery cells installed in the battery. The maximum output value and maximum input value of the battery are managed by the maps.
When the output voltage of the battery cell becomes smaller than its lower limit value (when the output voltage of the battery cell becomes a cut-off voltage), there occurs a situation in which the battery cell fails. When the input voltage of the battery cell becomes larger than its upper limit value (when the input voltage of the battery cell becomes a cut off voltage), there occurs a situation in which the battery cell fails.
The vehicle control unit stops supplying electric power to the motor when detecting from detection results of output voltage values and input voltage values of the respective battery cells by the respective voltage sensors that the output voltage values of the respective battery cells become their lower limit values (their cut-off voltages). In addition, the vehicle control unit stops supplying electric power to the respective battery cells.
On the other hand, the battery cells deteriorate as a result of being used. Specifically, the maximum output values and maximum input values of the battery cells at respective charging rates and temperatures are reduced.
The deterioration of the battery cells produces a difference between the maps which show the maximum output value and maximum input value of the battery and the actual performance of the battery (the maximum output electric power value that can be outputted by the battery and the maximum input electric power value that can be inputted into the battery).
It is considered that even in such a state that the maximum output value or maximum input value has not yet been reached on the map, by the deterioration of the respective battery cells, the maximum output electric power values or maximum input electric power values of the respective battery cells reach the cut-off voltage value (the output voltages become smaller than the lower limit value or the input voltages become larger than the upper limit value). Because of this, it is not preferable that there is produced a difference between the performance of the battery shown on the maps and the actual performances of the respective battery cells (the actual performance of the battery).
To cope with this, it has been practice to optimize the maps to mach the deteriorated battery. As one of the related arts of this type, the optimization of the maps is implemented by estimating deteriorations of the respective battery cells from a length of time when the battery has been used (for example, refer to JP-A-2004-6191).
When looking into causes for deterioration of the respective battery cells, the battery is affected by time over which the battery has been used and environment in which the battery is used. Temperature can be raised as one of causes related to the environment.
However, JP-A-2004-6191 discloses the related art for estimating deteriorations of the respective battery cells from time over which they have been in use. Because of this, depending upon an environment where the battery is used, it is considered that the deterioration of the battery cells is developed to a further advanced state than the estimated deterioration thereof from the time.
In such a state, since there is being produced a difference between the performance shown in the maps and the actual performance of the battery, it is considered that the output voltage values and input voltage values of the respective battery cells become the cut-off voltage value frequently, and every time the cut-off voltage is reached, the motor of the electric vehicle is stopped.